The U.S. Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA) have released a final rule that promises to significantly lower greenhouse gas emissions from medium- and heavy-duty vehicles, while creating new technological and compliance challenges for engine manufacturers and fleet owners.

The new "Phase 2" regulations build on "Phase 1" fuel efficiency standards for medium- and heavy-duty trucks enacted in 2011. The Phase 1 standards apply to new trucks and heavy vehicles in model years 2014 and later and are based on the use of "off-the-shelf" technologies that were already in production at the time of the rulemaking.

The Phase 2 program goes beyond existing technologies and sets standards that are intended to spur the adoption of new emission reduction technologies that are currently either under development or not yet widely deployed. Emissions reductions will be phased in over a nine year period, beginning in model year 2018.

The Phase 2 regulations set emission standards for diesel and gasoline engines that aim to achieve a 4 to 5% improvement in fuel efficiency and carbon dioxide emissions relative to model year 2017 engines. To achieve this goal, EPA anticipates that engine manufacturers will continue the adoption of Phase 1 technologies and increase the use of waste heat recovery systems for tractor engines.

The Phase 2 regulations also seek to achieve further reductions in emissions through tractor design standards, which will rely on improvements in the transmission, driveline, aerodynamic design, tire rolling resistance, and idle performance. In addition, for the first time, carbon dioxide emissions limits will be imposed on new trailers, which will be required to adopt new technologies such as low resistance tires, tire pressure systems, and aerodynamic improvements.

The federal effort to reduce greenhouse gas emissions from trucks and other medium- and heavy-duty vehicles parallels similar efforts in California, which uniquely among U.S. states has the authority to adopt its own emission standards, and at the federal level in Canada. In the preamble to the final rule, EPA and NHTSA note the strong interest of the regulated community for harmonizing the Phase 2 regulations across the U.S. and Canada. The California Air Resources Board (CARB) reacted positively to the announcement of the Phase 2 requirements, but the relationship between the state's greenhouse gas emission reduction goals and the federal standards remains an open question.